In fluid systems, actuators such as hydraulic cylinders or motors are often controlled using directional valves. FIG. 1 shows a conventional system of this type. The directional valve shown in this figure is a spool valve 11 that can be mechanically or electrically actuated to supply fluid from a pump 12 to one chamber of a cylinder 13 and drain fluid from a second chamber to a tank, or reservoir 14. In the subsequent text, the term “tank” is used to indicate a container or reservoir where oil is collected for reuse. In order to avoid flow in the wrong direction, in this case from the cylinder 13 to the pump 12, a non-return valve 15 is provided in the conduit supplying fluid to the cylinder. The non-return valve may comprise a ball or a cone held in contact with a seat by a resilient spring. This type of non-return valve is very robust and is generally considered to be fail-safe in a fluid system to prevent fluid flow in a particular direction.
A conventional valve controlled system of the above type suffers from energy losses caused by, for instance, a pressure drop across different valve components and the fact that pressurized fluid is drained to a tank. In order to control an actuator under load in a more energy efficient way, a valve arrangement may be provided with separate controllable valves, as shown in FIG. 2. In this figure the single directional valve has been replaced by four separate 2-port valves 21a-21d. 
FIG. 2 shows a schematic representation of a valve arrangement for controlling the boom of a crane. The 2-port valves 21a-21d are electrically operated valves controlled by a central processing unit, also termed a CPU. A first supply valve 21b is controlled to supply fluid from a controllable pump 22 to a first chamber of a cylinder 23 in order to raise or lower a crane arm C. A first drain valve 21d is controlled to drain fluid from a second chamber of the cylinder 23 to a tank 24 as the crane arm C is being raised. A second supply valve 21c is controlled to supply fluid from the controllable pump 22 to the second chamber of the cylinder 23 in order to lower a crane arm C. A second drain valve 21a is controlled to drain fluid from the first chamber of the cylinder 23 to a tank 24 as the crane arm C is being lowered. A back-pressure valve 25 is provided in the conduit connecting the valves 21a-21d to the tank 24. In order to minimize energy losses, the separate valves 21 a-21d are controlled by the CPU based on control signals from an operator and signals from a number of pressure sensors 26, 27, 28 indicating the fluid pressure in various parts of the system. The control and sensor input signals allow the CPU to continuously calculate an optimized control strategy for controlling the pump and the valves.
Under certain load condition, an optimum control includes returning at least a part of the fluid flow to the pump, whereby the pump acts as a motor to recover energy that may be used in other parts of the system. Consequently the valves located between the pump and the cylinder ports must allow for flow control in both directions.
In order to minimize pressure losses during, for instance, a lifting movement the pressure drop over each valve must be relatively low. In many cases the pressure drop may be only a few percent of the absolute pressures. It is important that a system of this type is fail-safe. However, if one or more of the pressure sensors transmits an incorrect signal, indicating a pressure that is a few percent higher or lower than the actual pressure, a desired lifting movement may instead result in a sudden drop of a load carried by the crane arm. From a safety point of view, this is unacceptable. Even if redundant sensors are used it may be difficult to ensure that the system is fail safe, as the pressure delivered by the pump and the load on the cylinder can change rapidly. Consequently, a load being raised may suddenly begin to drop if an error has developed in the system.
An object of this invention is to provide a fail safe valve arrangement allowing control of the flow of fluid pressure medium in one direction or the other in a conduit with fluid pressure medium, which conduit may act as a supply as well as a return conduit between a pressure source and a consuming device. In addition, the valve arrangement should have a simple and reliable function avoiding the above problems.